Pregnancy Check-ups, Yolk Sac, and Its Functions

After a woman becomes pregnant, she needs to regularly visit the hospital for prenatal check-ups. These check-ups not only reveal the condition of the fetus but also provide information about the mother's status. When a woman is six weeks pregnant, the yolk sac usually appears during the examination. However, some pregnant women may not have a yolk sac during their check-ups. This can be attributed to two main reasons: the influence of the woman's menstrual cycle and poor embryonic development. It is advisable for these women to undergo further examinations.

Normal Absence of Yolk Sac at Six Weeks

Firstly, it is normal for there to be no yolk sac visible at six weeks of pregnancy. It is recommended to recheck at eight weeks. Typically, the yolk sac can be seen through a B-ultrasound scan around the sixth week of pregnancy. However, some expectant mothers may not see the yolk sac at this stage due to late conception or slow fetal development. Therefore, if no yolk sac is visible at six weeks, there is no need to worry. It is advisable to recheck at eight weeks, where the yolk sac and fetal heartbeat can be observed, indicating normal fetal development.

Formation and Structure of the Yolk Sac

Secondly, the yolk sac is formed after the embryo's body folds and the primitive gut divides into the endodermic gut and the extraembryonic yolk sac. The yolk sac contains a large amount of yolk, and its wall is formed by the extraembryonic endoderm and mesoderm. The yolk sac is surrounded by a rich membrane sac in the abdominal cavity and is connected to the embryonic midgut through a narrow stalk called the yolk stalk. The yolk sac precedes the formation of the embryonic bud and the fetal heart, and it is usually visible when the gestational sac can be seen. The presence of the yolk sac indicates that the embryo can continue to develop.

Functions of the Yolk Sac

Thirdly, the yolk sac plays crucial roles in early embryonic development. It serves as an important transport system between the embryo and the mother. The yolk sac is the first structure to form after the embryo's body folds, and it contains a large amount of yolk that provides nutrition to the developing embryo. The walls of the yolk sac are formed by layers of cells that will eventually give rise to various tissues and organs. The presence of the yolk sac during early pregnancy is a sign that the embryo is developing normally.

Importance of Yolk Sac Detection

Furthermore, the appearance of the yolk sac in the gestational sac is considered normal, and the absence of the yolk sac may indicate a potential issue with the pregnancy. In a metaphorical sense, the presence of the yolk sac is akin to having a house ready for habitation. Typically, the yolk sac can be detected around 35 days after conception, and within 1-2 weeks of its appearance, the embryonic bud and fetal heartbeat can also be observed using a B-ultrasound scan.